Several different techniques have been used in order to clean the interior of piping systems. These clean-in-place techniques include pigging, brush cleaning, lances and fluid flow or hydrodynamic cleaning. Pigging and brush cleaning require direct physical contact of a tool with the interior of the pipe. Pigging, brush cleaning and lance cleaning techniques can all be time consuming and require special equipment. For that reason, hydrodynamic cleaning is generally the method of choice for cleaning operations which require quick turn around time and/or relatively low cost.
In the case of single-phase flow of liquids for hydrodynamic cleaning, high liquid flow rates are needed to achieve effective cleaning. The resulting flow is not particularly efficient for cleaning and is wasteful in terms of the volume of liquid used for cleaning. Through the use of two-phase flow in cleaning operations, the amount of liquid waste is decreased and a more efficient hydrodynamic cleaning is accomplished. However, improper application of two-phase flow in cleaning operations can also result in an ineffective cleaning or even in a compounding of the cleaning problem. If the gas flow used in the two-phase flow is insufficient, the flow will more resemble a single-phase flow and, thus, the cleaning will be insufficient. If too much gas flow is used in a two-phase flow cleaning operation, there is the potential of that the fouling within the piping will be dried and hardened making it more difficult to remove.
Two-phase flow for use in cleaning operations is known. One example is taught in U.S. Pat. No. 4,161,979 to Stearns. Stearns actually teaches a method for flushing an automobile cooling system using a mixture of water and pressurized air. Although Stearns teaches gauges and valves in order to monitor and control the flow rates of air and water to the mixing chamber, he does not seem to suggest any particular air/water ratio regardless of the system being cleaned.
In an article entitled "Mechanical Cleaning Effect and Pressure Drop of Air-Water-Flow in Horizontal Glass Tubes (Vacuum Dairy Pipelines)" which appeared in a 1980 issue of Journal of Food Process Engineering 3, mathematical models were presented for calculating pressure drop and mechanical cleaning effect for plant design of vacuum dairy milking pipes. In such article, two-phase flow was reviewed for its ability to clean glass tubes. The two-phase flow was generated with the aid of a vacuum system. Water was pumped into the piping system and air was drawn through with the vacuum system, with both flows being controlled by valves. Tradardh and VonBockelmann, authors of the article, correlated their cleaning results to that of the predicted two-phase pressure drop, using the Dukler Homogeneous Pressure Drop Model. In this correlation, the slopes of the constant cleaning effect curves were related to the constant pressure drop curves. By this approach it was shown that the slopes were nearly identical for annular flow (high air to water flow ratios) and the slopes began to deviate in the slug flow regime (low air to water flow ratios). It was concluded that at low air fractions, that is, low air to water ratios, there was an increase in the pressure gradient but there was not a corresponding increase in cleaning efficiency. Alternatively, with high air fractions there was both an increase in pressure gradient and an increase in cleaning efficiency. Thus, such article concludes that cleaning efficiency increased with both pressure and flow rate ratio.
The prior art fails to recognize in that for cleaning efficiency, there is an optimum flow rate ratio for any particular piping system. The prior art fails to teach any method for arriving at this optimum flow rate ratio and further fails to recognize that cleaning efficiency resulting from two-phase flow may vary at different points within the piping system.